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Yeah Theo - I agree re sensitivity and I am at fault for inferring it does improve the ability to capture fainter "thingos" - it is just a reference to the instruction manual.
For the guys that dont want acronyms and overly technical talk - Theo is pointing out that the ability of the pixel to capture a quantum (ie a 'bit or packet) of light and convert it to voltage (ie the charge generation) is in no way changed by binning. The ability of the pixel to do this is fixed in the physical properties and performance of the electronics that forms the little bucket that is the pixel.
We teach the undergrads at uni that a CCD is like a series of rows of small buckets (ie pixels) that catch rain (light quanta or packets). By putting 4 buckets together you in no way effect the ability however of each of the individual buckets to catch and retain rain (QE or Quantum Efficiency) by adding them together (binning). It is a good analogy for describing a CCD and I have seen it even being used in introductory text books. It can even be taken further to include dumping the buckets into tanks (serial registry) and then measuring the content of each tank - you can obviously do this quicker (ie faster download speed) if you are dumping 4 buckets at a time instead of 1 at a time. Just another quick one is that each bucket has a capacity (full well capacity). It obviusly is more tech than this (and definately gets beyond me very quickly with regard to most of it) but it may be of use to understand some of the tech talk that goes on about CCDs.
Mark
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